This invention relates to discrete, shaped blanks of plasticized polyvinyl butyral (PVB) suitable for laminated safety glass windows and more particularly to a special design of such blank to accommodate a head-up feature associated with a windshield.
Thin sheet of plasticized PVB is well known as interlayer in laminated safety glass finding application in windows such as penetration-resistant automotive and aircraft windshields. The sheet used in modern windshields having severely curved and angled configurations has in the past been shaped by stretching straight-sided sections of unrolled sheet to the desired windshield configuration before glass lamination. When the interlayer has a color band along one margin intended to reduce glare from the sun when in place in the windshield, as is usually the case, stretching is such as to cause the initially straight color band to assume an arcuate configuration which matches the curved shape of the windshield. U.S. Pat. No. 4,808,357 discloses an improved apparatus and process for forming such a windshield blank by stretching initially uneven, extruding sheet the desired amount while the plastic is at elevated temperature and less likely to develop stresses causing shrinkback during later glass laminating. Blanks formed by such a system also have good thickness uniformity leading to improved impact resistance in service in the glass laminate.
Head-up display systems using holographic optical elements have recently been proposed for use in vehicle windshields. With these systems information is displayed to an occupant of the vehicle such as its driver while permitting such occupant to look straight ahead through the windshield. Head-up displays eliminate the need to glance downwardly to view operating information displayed on dials on the dashboard panel. However, at least with some head-up display systems using holograms, if the glass layers laminated on either side of the hologram assembly forming a conventional safety glass windshield are parallel to each other, as would be the case using blank of uniform thickness formed according to the '357 patent, the head-up display system will not function properly. In order for such head-up display system to function properly, the glass layers in the windshield laminate can not be parallel. It would be desirable to provide an interlayer windshield blank tailored to accommodate this developing requirement.